Shorter code: remove unnecessary condition

See https://github.com/rails/rails/commit/136fc65c9b8b66e1fb56f3a17f0d1fddff9b4bd0#r28897107

 I _think_ that this method can now be rewritten from:

```ruby
def attribute_previous_change(attr)
  previous_changes[attr] if attribute_previously_changed?(attr)
end
```

to:

```ruby
def attribute_previous_change(attr)
  previous_changes[attr]
end
```

without losing performance.

---

Calling

```ruby
previous_changes[attr] if attribute_previously_changed?(attr)
```

is equivalent to calling

```ruby
previous_changes[attr] if previous_changes.include?(attr)
```

When this commit 136fc65c9b was made, Active Record had its own `previous_changes` method, added here below. However, that method has been recently removed from the codebase, so `previous_changes` is now only the method defined in Active Model as:

```ruby
def previous_changes
  @previously_changed ||= ActiveSupport::HashWithIndifferentAccess.new
  @previously_changed.merge(mutations_before_last_save.changes)
end
```

Since we are dealing with a memoized Hash, there is probably no need to check `if .include?(attr_name)` before trying to fetch `[attr]` for it.

Does that make sense? Did I miss anything? Thanks!

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